1. Field of the Invention
The present invention relates to a controller for a wire electric discharge machine, and particularly to a controller for controlling the wire electric discharge machine to perform finish machining of a second cut and subsequent cuts.
2. Description of Related Art
In the wire electric discharge machine, as methods of controlling the feeding of a wire electrode relative to a workpiece, a constant average machining voltage method and a constant feed speed method are known. The constant average machining voltage method controls the feed speed such that the average machining voltage across a gap between the workpiece and the wire electrode is constant, in order to increase the speed of first cutting, namely contour cutting first performed on the workpiece, and prevent the break of the wire electrode due to concentrated electric discharges. Hence, the constant average machining voltage method is not always suitable for finish machining after the first cutting, namely second cutting and subsequent machining, in which electric discharge machining is performed using a smaller electric discharge pulse current to improve surface roughness and accuracy. When the constant average machining voltage method is adopted, the feed control is performed while making various adjustments to minimize change in the amount of machining per unit time to stabilize electric discharge pulse density.
For example, the following technique for improving the machined surface roughness is known: In first cutting, when the average machining voltage becomes equal to a reference voltage value, or in other words, differential voltage becomes zero, feeding is stopped, and when the differential voltage becomes opposite in sign, feeding in the opposite direction is performed. In medium finishing in finish machining, when the average machining voltage becomes equal to the reference voltage value, namely the differential voltage becomes zero, control is performed such that feeding is performed at a set feed speed. In finishing after the medium finishing, the power source is switched to a high-frequency power source and the servo control proportional gain is made smaller (see JP 3231567B).
In the constant average machining voltage feed control, feeding cannot be performed with sufficient accuracy, since change in average machining voltage cannot reflect change in removal width (width of a part to be removed) accurately. Further, it is very difficult to choose a proper gain according to chance in average machining voltage corresponding to change in removal width. Hence, the conventional control has a problem that it cannot provide stable surface accuracy repeatedly. Also in the constant feed speed machining, it is difficult to improve the accuracy of machining.
Under the circumstances, the applicant has proposed a controller for a wire electric discharge machine which keeps the amount of machining constant without requiring the adjustment of gain, etc. in control of feeding a wire electrode relatively to a workpiece, to thereby provide stable machining and high surface accuracy, which has become publicly known as JP 2004-283968A.
FIG. 6 is a diagram for explaining how irregularities of a machined surface are made flat by finish machining to improve surface accuracy.
In FIG. 6, reference sign 5 indicates a workpiece, and reference sign 4 a wire electrode. In order to achieve the purpose of finish machining, namely to make the irregularities of the machined surface of the workpiece 5 flat, the amount of feeding should be changed depending on change in removal width so that the amount of machining is constant. In FIG. 6, provided that ts (=t) and tx (=t) are board thicknesses, Gs and Gx are widths of portions to be removal, and δs and δx are amounts of movement per unit time T,δs*ts*Gs=δx*tx*Gx∴δx=δs*(ts/tx)*(Gs/Gx)  (1)
Here, as shown in FIG. 8, the area that undergoes electric discharges (referred to as “electric discharge area”) S1, S2 as shown in FIG. 7 is proportional to the number of times that electric discharges are repeated (referred to as “number of repetitions of electric discharge”) F1, F2 while unit distance δ is traveled. In other words, the number of repetitions of electric discharge F1, F2 is proportional to the amount of machining. Since the amount of machining per unit distance corresponds to the electric discharge area S, namely the board thickness t multiplied by the width G of the portion to be removed, the relation between ts*Gs, tx*Gx and the numbers of repetitions of electric discharge Fs, Fx is:(ts*Gs)/(tx*Gx)=K*(Fs/Fx)∴(ts/tx)*(Gs/Gx)=K*(Fs/Fx)  (2)
K: constant determined by machining conditions
Here, provided that δs is a reference motion amount, the reference motion amount is obtained from a reference feed speed SPDs which is preset and entered, as follows:δs=SPDs*T  (3)From equations (1), (2) and (3), the motion amount δx is:δx=SPDs*T*K*(Fs/Fx)  (4)Since δx/T=SPDx,SPDx=K*SPDs*(Fs/Fx)  (5)
Thus, the speed SPDx corresponding to the motion amount δx in the unit time T in the portion of the width Gx is the reference speed SPDs multiplied by the ratio Fs/Fx between the number of repetitions of electric discharge Fs in the portion of the width Gs of the part to be removed and the number of repetitions of electric discharge Fx in the portion of the width Gx. This means that when the number of repetitions of electric discharge Fx is obtained, the speed SPDx which makes the amount of machining equal to the reference amount of machining can be obtained on the basis of the ratio Fs/Fx, where the reference number of repetitions of electric discharge Fs can be obtained in advance. In other words, the ratio Fs/Fx is a value proportional to the amount of machining. Hence, feeding can be performed such that the amount of machining is constant, according to change in removal width.
Further, provided that average no-load time in the part where the motion amount is the reference amount δs is TW(S), average no-load time in the part where the motion amount is δx is TW(X), set no-load voltage is Vp, and pause time is TOFF, average machining voltages Vs and Vx in the respective parts are:Vs=Vp*TW(S)/(TW(S)+TOFF)  (6)Vx=Vp*TW(X)/(TW(X)+TOFF)  (7),where electric discharge time TON is ignored since it is very short.
In this case, the numbers of repetitions of electric discharge Fs, Fx in the respective parts are:Fs=1/(TW(S)+TOFF)  (8)Fx=1/(TW(X)+TOFF)  (9)Eliminating the average no-load times TW(S) and TW(X) from equations (6), (7), (8) and (9) givesFs*TOFF=(Vp−Vs)Vp  (10)Fx*TOFF=(Vp−Vx)Vp  (11)When (Vp−Vs) is substituted by average drop voltage Es in standard machining and (Vp−Vx) is substituted by average drop voltage Ex, the above equations indicate that the average drop voltage Es, Ex is proportional to the number of repetitions of electric discharge Fs, Fx, and hence, the amount of machining.
Substituting equations (10) and (11) into equation (5) givesSPDx=K*SPDs*Es/Ex  (12)
Thus, the speed SPDx corresponding to the motion amount δX is the reference speed SPDs multiplied by the ratio between the average drop voltage in the part where the motion amount is the reference amount δs and the average drop voltage in the part where the motion amount is δx. This means that even when the number of repetitions of electric discharge cannot be obtained, the same result can be obtained from the no-load voltage and the average machining voltage as that obtained from the number of repetitions of electric discharge according to equation (5). The no-load voltage is a predetermined voltage and known. Hence, when the average machining voltage is detected, the average drop voltage Ex, and the ratio (Es/Ex) between the average drop voltage Es in standard machining and the average drop voltage Ex can be obtained. From this ratio and the reference speed SPDs, the speed SPDx which makes the amount of machining equal to the reference amount of machining can be obtained. Thus, feeding can be performed such that the amount of machining is constant, according to change in removal width.
On the basis of the above, JP 2004-283968A has proposed a controller for a wire electric discharge machine as shown in FIG. 9.
In FIG. 9, an electric discharge pulse generator 1 is for applying an electric discharge pulse voltage across a gap between a wire electrode 4 and a workpiece 5 for electric discharge machining, and comprises a direct-current power source, a circuit including a switching element such as a transistor, a capacitor charging/discharging circuit, etc. Conducting brushes 2 and 3 are for transmitting electricity to the wire electrode, and connected to one of the terminals of the electric discharge pulse generator 1. The workpiece 5 is connected to the other terminal of the electric discharge pulse generator 1. The electric discharge pulse generator 1 applies a pulse voltage between the traveling wire electrode 4 and the workpiece 5. A table (not shown) on which the workpiece 5 is mounted is driven by an X-axis drive motor control device 10, a Y axis drive motor control device 11 and a feed pulse distribution unit 12 which constitute movement means.
FIG. 9 relates to an example in which the average drop voltage Ex is detected as a physical quantity for detecting the amount of machining. Hence, a discharge gap detection unit 6 is connected with the wire electrode 4 and the workpiece 5, detects pulses of gap voltage having a length of several μs or shorter coming from electric discharge pulse generator 1 and supplies the detected values to a machining rate detection unit 7. The machining rate detection unit 7 is for obtaining the average Ex of drop voltage values, namely differences between a set no-load voltage and pulses of gap voltage, in a predetermined period (unit time) T, on the basis of a signal sent out from an arithmetic clock 14 at intervals of the predetermined period (unit time) T, and constitutes machining rate determining means for determining the machining rate. A reference machining rate storage unit 8 is for storing a drop voltage value Es corresponding to a reference rate of machining which is inputted in advance.
In the reference machining rate storage unit 8, a drop voltage value Es corresponding to a predetermined reference rate of machining is stored. A comparison/determination unit 9 compares the average drop voltage value Ex in the unit time (predetermined period) T obtained by the machining rate detection unit 7 and the drop voltage value Es corresponding to the reference rate of machining supplied from the reference machining rate storage unit 8, at intervals of the unit time (predetermined period) T, and supplies the ratio (Es/Ex) between the drop voltage value Es corresponding to the reference rate of machining and the average drop voltage value Ex, to a feed pulse calculation unit 13.
On the basis of the signal sent out from the arithmetic clock 14 at intervals of the unit time (predetermined period) T, the feed pulse calculation unit 13 obtains the motion amount δx by multiplying a distance (SPD*T), which is obtained from a feed speed SPD supplied from feed speed setting means 15 and the predetermined period T, and the ratio (Es/Ex) between the drop voltage value Es corresponding to the reference amount of machining and the average drop voltage value Ex, which is supplied from the comparison/determination unit 9, and sends a train of pulses corresponding to the motion amount δx to the feed pulse distribution unit 12. On the basis of the train of pulses supplied, the feed pulse distribution unit 12 supplies X-axis drive pulses and Y-axis drive pulses to the X axis drive motor control device 10 and the Y-axis drive motor control device 11 according to machining programs, to drive an X-axis motor and a Y-axis motor for driving the table on which the workpiece 5 is mounted.
With the controller arranged as described above, finish machining can be performed with the constant amount of machining, stably and with high surface accuracy.
However, there remains a problem. In finish machining in wire electric discharge machining, namely second cutting and subsequent machining, normally, a part of several μm to several tens μm in width is removed from a machined surface of a workpiece. However, as shown in FIG. 10, in a region that forms a corner (referred to as “corner portion”), the part Q that remains to be removed after previous machining (referred to as “remaining part”) has a width much greater than that in a straight-line portion. In some cases, a remaining part Q of several hundreds μm in width needs to be removed. The technique disclosed in JP 2004-283968A is a control method for improving the surface roughness and accuracy by removing a part of several μm to several tens μm in width from the machined surface. However, in a region such as a corner portion in which the amount of machining is very large, it is difficult to perform control on the basis of the normal ratio of the distance of movement to the amount of machining. In such case, machining is unstable such that short circuits are produced repeatedly, and cannot provide high form accuracy.